This invention concerns filters and more particularly filters adapted to filter melted plastic in injection molding machines.
It often occurs that melted plastic becomes contaminated with various foreign objects and material such as scrap metal, floor sweepings, etc., and the presence of such foreign objects in the melted plastic can lead to loss of production due to their containment in the molded parts, and also create other problems as by lodging of the particles in the mold or cavities, tending to block flow.
There has thus heretofore been provided filters to filter the plastic melt prior to injection into the mold. A known filter design consists of a filter screen plug having a central cavity which receives the melt and is also provided with an axial series of tapered filter holes. The melted plastic passes through the holes and into an outer annular discharge chamber. The filtered melt is then collected and redirected into a central passage in communication with the injection passage of a nozzle. Examples of this design are shown in U.S. Pat. Nos. 3,767,056 and 3,825,123.
There are also injection molding filters which employ flat screens to filter the plastic.
Such injection molding filters are in widespread use and do serve to greatly reduce the incidence and severity of problems associated with foreign material in the plastic melt.
Such arrangements, however, do inherently involve flow through relatively small diameter holes. That is, hole diameters on the order of 0.020 to 0.040 inches in diameter are typical depending on the degree of coarseness or fineness of the particular filtering action required.
Usage of such fine holes makes cleaning of the filter screen relatively difficult. The screen must periodically be cleaned in order to restore full-flow to the filter since the particles collected in the filter tend to plug up these holes.
An additional factor is the substantial pressure drop caused by filter holes interposed in the flow path to the injector nozzle, necessitating higher pressures to properly carry out injection.
A characteristic of the tubular type filter screens of the type disclosed in the aforementioned patents is a relatively large diameter annular clearance space between the tubular screen and an outer filter body. This relatively large diameter annular chamber is subjected to high injection pressures and necessitates a relatively large diameter main body to withstand these pressures, increasing the bulk of the filter unit.
The arrangement also involves a necessary recollecting of the flow in an annular passage so as to enable it to be directed into the injection nozzle passage, increasing the complexity of the components.
Another problem involved in the injection molding process is concerned with the need to maximize the homogeneousness of the melt. The melting plastic, depending on the source and the make-up of the feed stock, tends to vary in density. It has been found that the extent of linear or laminar flow to the point of injection tends to improve the uniformity of the melt and the resulting uniformity in density of the molded product. Turbulent or tortuous flow tends to aggravate the problem of lack of uniformity. The use of a large number of fine pores tends to induce turbulent flow and thus does not serve as an aid to improve homogeneousness, indeed tends to worsen the problem.
Accordingly, it is an object of the present invention to provide a filter for injection molding machines which avoids the use of very small diameter holes for the primary filtration process such as to minimize the pressure loss in the filter, as well as to allow easier cleaning of the filter.
It is yet another object of the present invention to provide a filter for injection molding machines in which a high degree of laminar flow through the unit is induced to improve the homogeneousness of the melted plastic.
It is still another object of the present invention to provide such filter which is of simple configuration such as to be able to be manufactured at relatively low cost.
It is still another object of the present invention to provide such a filter arrangement in which the filter size is minimized due to a reduction in pressure forces acting thereon over the previously heretofore known tubular type injection filters.